Some Researchers Thought This Drug Could Treat the Polio-Like Illness Sickening Children. A New Study Says It Probably Doesn’t

Health officials are scrambling for answers — and cures — as a rare, polio-like illness called acute flaccid myelitis (AFM) spreads across the country.

Some researchers thought the antidepressant fluoxetine, which is often sold under the brand name Prozac and has some antiviral properties, could work against AFM, a mysterious illness that strikes some children and can cause muscle weakness, breathing issues, paralysis and even death. A new study published Friday in Neurology, however, provides disappointing results: In a small, non-randomized retrospective study, fluoxetine did not seem to restore patients’ muscle strength.

The Centers for Disease Control and Prevention (CDC) has confirmed 80 cases of AFM in 25 states so far this year, but has yet to determine the cause of the outbreak. Experts do know, however, that AFM often follows other viral infections, including those from enteroviruses. Since fluoxetine has been shown to work against enterovirus D68, some doctors thought it might fight AFM, too.

The Neurology study, led by Dr. Kevin Messacar, an assistant professor of pediatrics at Children’s Hospital Colorado and the Colorado University Anschutz Medical Campus, suggests otherwise. Researchers looked back at the medical records of 56 young children who were diagnosed with AFM in 2015-2016. Twenty-eight of these kids received more than one dose of fluoxetine, while another 28 received either one or zero doses. The researchers looked at the results of muscle strength exams conducted during the kids’ illnesses to see whether the antidepressant was associated with any improvements.

While there was no significant difference in muscle strength during initial exams, over time, the children who took fluoxetine actually fared worse. After seven months, their strength scores had dropped by an average of 0.2 (on a scale of 0 to 20), while the non-treated children improved by an average of 2.5 — even though more than half of the patients who took the drug had enterovirus D68 in their systems. This trend remained even after adjusting for other health factors.

While the study had some limitations, given its small sample size and backward-looking design, it still suggests that doctors will have to keep looking for therapies that work against AFM. CDC officials told TIME in October that defining the cause of the outbreak, which would pave the way to finding treatments, remains a priority.

“While CDC has determined causes of some AFM cases, we don’t yet have a comprehensive understanding of most of the AFM cases we have investigated,” the statement said. “We don’t know what has been causing the outbreaks since 2014. We wish we knew more now and we’re working hard to find the cause or causes.”